The present invention relates to a programmable-gain amplifier making use of switching characteristics of MOS (Metal Oxide Semiconductor) transistors and transfer characteristics of bi-polar transistors.
FIG. 6 is a circuit diagram illustrating a conventional example of the programmable-gain amplifier making use of switching characteristics of MOS transistors and transfer characteristics of bi-polar transistors, which have a cascade connection of a first to an n-th (n being an integer more than one) differential amplifier E.sub.1 to E.sub.n, each having a circuit configuration disclosed in Japanese patent application laid open as Provisional Publication No. 139531/'96.
Referring to FIG. 6, each of the differential amplifier, the first differential amplifier E.sub.1, for example, comprises a pair of differential bi-polar transistors Q.sub.1 and Q.sub.2 having emitters commonly connected to a constant current supply I.sub.1, a pair of first resistors R.sub.11, a pair of second resistors R.sub.12, and a pair of MOS transistors M.sub.1 and M.sub.2. Each of the first resistors R.sub.11 and each of the second resistors R.sub.12 are serially connected between a power supply Vcc and a collector of each of the differential bi-polar transistors Q.sub.1 and Q.sub.2. Each of the MOS transistors M.sub.1 and M.sub.2 is connected to each of the first resistors R.sub.11 in parallel and gates of the MOS transistors M.sub.1 and M.sub.2 are commonly controlled by a first control signal CTL.sub.1.
When the MOS transistors M.sub.1 and M.sub.2 are controlled to be OFF, the amplification factor A.sub.1 of the first differential amplifier E.sub.1 is represented by EQU A.sub.1 =q.times.I.sub.1 .times.(R.sub.11 +R.sub.12)/2kT, (1)
and is represented by EQU A.sub.1 =q.times.I.sub.1 .times.(R.sub.11 +R.sub.0)/2kT (2)
when the MOS transistors M.sub.1 and M.sub.2 are controlled to be ON, where q, k, T, I.sub.1, R.sub.11, R.sub.12 and R.sub.0 represent the charge of an electron, the Boltzman constant, an absolute temperature, a current value of the constant current supply I.sub.1 and resistance values of the first resistor R.sub.11, the second resistor R.sub.12 and the parallel connection of the first resistor R.sub.11 and on-resistance of the MOS transistor M.sub.1 or M.sub.2, respectively.
Hence, by controlling ON/OFF of the MOS transistors M.sub.1 and M.sub.2 by the first control signal CTL.sub.1, the gain, or the amplification factor A.sub.1, of the first differential amplifier E.sub.1 can be changed. In the same way, each of the gains of the second to the n-th differential amplifiers E.sub.2 to E.sub.n can be controlled by changing the logic level of each of a corresponding one of a second to n-th control signal CTL.sub.2 to CTL.sub.n.
Thus, the total gain A of the conventional programmable-gain amplifier of FIG. 6, which is given by a product of each of the amplification factors of the first to the n-th differential amplifiers E.sub.1 to E.sub.n, can be controlled with multi-steps according to the logic levels of the first to n-th control signals CTL.sub.1 to CTL.sub.n.
However, as can be understood from the above equation (2), the amplification factor of each differential amplifier E.sub.1 to E.sub.n is a function of the absolute value of the on-resistance of the MOS transistor in the conventional programmable gain amplifier of FIG. 1. Furthermore, the on-resistance of the MOS transistor is easily affected by variation of its diffusion process and depends on operating temperature.
Therefore, it has been very difficult to obtain a multi-step programmable-gain amplifier in which the gain can be controlled with sufficiently fine and linear steps in dB (decibels), that is, exponentially.